LlCloud

Caveats

 
default
{
    touch_start(integer num)
    {
        llSay(0,"Cloud density:" + (string)llCloud(ZERO_VECTOR));
    }
}

How clouds work:

SL features a cellular automata-based weather system.
There are 16x16 square cloud "cells" per sim (which means each is 16x16m in size).
The cloud density data is sent from the server to the client once every time the agent connects to a simulator via logging in or teleporting into a simulator.
The client attempts to render clouds from the cloud algorithm results sent from the server, which is why different clients can see different cloud formations and de-synchronizes cloud view from the server over time.
The Z value in the vector is irrelevant (clouds are always between about 180-200m absolute height).
It returns a value between 0.0 and 1.0, with 0.0 being no clouds, and 1.0 very dense clouds.
Values returned above 1.0 - 2.0 indicate rainfall, but the option for rain is not currently implemented so values are capped at 1.0.
Clouds and wind are related.

The cloud seed/growth/dissipation algorithm uses a solenoidal vector field, which is a vector field with zero-divergence: ∇ · v = 0

This condition is satisfied whenever v has a vector potential A, because if v = ∇ ⨯ A then ∇ · v = ∇ · (∇ ⨯ A) = 0

This article wasn't helpful for you? Maybe the related article at the LSL Wiki is able to bring enlightenment.

•	llWind
•	llGetSunDirection